The present invention generally relates to the utilization of an ion-sensitive field-effect transistor (FET) and, more particularly, to a method and device for compensating variation in characteristic of an ion-sensitive FET transducer which would result from change in temperature.
As an instrument for the measurement of ion activities in electrochemical and biological environments, an ion-sensitive electrode probe, or an ion sensor as it is generally called, has heretofore been largely employed. An ion-sensitive glass electrode used in most pH detecting devices is a typical ion-sensitive electrode probe. When in use, the ion sensor is directly immersed in a liquid of interest of which the ion activity is desired to be measured, and the measurement of the ion activity can be carried out merely by detecting a potential difference between the ion sensor and a reference electrode. With this ion sensor, a continuous measurement of the ion activity may also be possible. In view of this, the ion sensor is useful and has many applications particularly in a medical field.
However, when it comes to the manufacture of the ion sensor in such a compact and miniature size that it can be used in the measurement, and continuous monitoring of the measurement, of the ion activity in a local area of a tissue of a living body, an output impedance of the glass electrode tends to increase to such an extent that an electrical insulation can hardly be attained with a retarded responsivity.
In order to obviate the above described problem, an ion-sensitive FET transducer comprising a combination of an ion-sensitive electrode and a metal oxide semiconductor field-effect transistor (MOSFET) which serves as a preamplifier has recently been developed. The principle of this ion-sensitive FET transducer and its application for the measurement of the ion activity in a liquid of interest, for example, an electrolyte, are respectively illustrated in FIGS. 1 and 2 of the accompanying drawings, reference to which will now be made for the discussion of the prior art.
Referring first to FIG. 1, the ion-sensitive FET transducer 1 shown therein is of a construction wherein a gate insulating layer 2 is, in place of the gate metal, formed on a channel 5 defined between a source 3 and a drain 4. When in use in the measurement of the ion activity in the electrolyte 6 using a drain-grounded circuit shown in FIG. 2, the potential at the interface between the surface of the gate insulating layer 2 and the surface of the electrolyte 6 varies depending on the activity of a particular ion in the electrolyte 6, as is the case with the potential at the interface between the surface of the glass electrode and the electrolyte. Accordingly, if the potential of the electrolyte 6 is made fixed by using a reference electrode 7, change in potential at the interface between the gate insulating layer 2 and the electrolyte 6 results in change in electroconductivity of the channel 5 situated immediately below the gate insulating layer 2. Therefore, when the measurement of the ion activity is carried out by detecting the potential at the interface between the surface of the gate insulating layer and the surface of the electrolyte in the manner described above, unlike that with the use of the glass electrode, the electrode resistance can be neglected on the one hand and, on the other hand, since the output impedance of the FET is low, such an amplifier of high input resistance as heretofore required in electrical connection with the glass electrode is no longer needed.
The ion-sensitive FET transducer referred to above has the following features.
(1) Since the electrode resistance can be neglected, its miniaturization in size is faciliated and, the responsivity in measurement system is high. PA0 (2) Since any existing IC technology can be used in the manufacture of the ion-sensitive FET transducer, various ion sensors can be integrated in a compact size. PA0 (3) The gate insulating layer 2 can be made in a multilayered structure and the layer thickness can be controlled precisely within 100 A. In other words, since the selectivity of the ion-sensitive FET transducer to ions in the electrolyte is determined by the composition of the surface of the gate insulating layer 2, various types of sensors selectively sensitive to ions can be manufactured merely by suitably selecting the composition of the gate insulating layer 2 which is a layer sensitive to ions. With this ion-sensitive FET transducer, the potential at the interface between the gate insulating layer and the electrolyte or any other liquid of interest is governed by the electrochemical equilibrium as is the case with a usual ion sensitive electrode selectively sensitive to ions.
This ion-sensitive FET transducer described above was first disclosed by Piet Bergveld, IEEE Transactions of Biomedical Engineering, 1970, (Vol. BME 17) and does not make use of the reference electrode during the measurement. However, subsequent to the publication of the ion-sensitive FET transducer by Piet Bergveld, IEEE Transactions of Biomedical Engineering, 1978, (Vol. BME 25) discloses a system of measurement using a combination of the ion-sensitive FET transducer with the reference electrode devised by Matsuo, et al. At the same time, this paper discloses that an ion-sensitive FET transducer of a construction wherein the gate insulating layer is made of silicon nitride (Si.sub.3 N.sub.4) exhibits an ion selectivity similar to or as comparable to the glass electrode for the pH measurement.
Since then, various attempts have been made to develop improved versions of the ion-sensitive FET transducer including those selectively sensitive to H.sup.+, Na.sup.+, K.sup.+, Ca.sup.++, .div.Ag.sup.+, Cl.sup.-, F.sup.- and other ions, and even research is nowaday conducted to develop an ion-sensitive FET transducer selectively sensitive to hydrogen gas.
Where the ion activity measurement is to be performed by the use of the ion-sensitive FET transducer, the ion-sensitive FET transducer 1 is, as shown in FIG. 2, immersed in the electrolyte solution 6 contained in a vessel or container 8 with the drain 4 and the source 3 electrically connected respectively to a constant voltage source +Vd and a constant current supply device 9. The constant current supply device 9 is so adjusted that the drain current Id can always be fixed. During the measurement, the voltage V indicated by a potentiometer 10 is expressed as follows. EQU V=Eg+Es-Er (1)
wherein Eg, Er and Es represent the gate potential of the ion-sensitive FET transducer 1, the electrode potential of the reference electrode 7 and the potential of the source 3 relatively to the gate, respectively.
Since the potentials Eg and Er tend to be affected by change in temperature and the magnitude of change in characteristic as result of change in temperature varies from one ion-sensitive FET transducer to another, an accurate and precise measurement of the ion activity in the electrolyte solution 6 tends to be hampered.
This will be discussed in more detail. Let it be assumed that the concentration of ions in the electrolyte solution 6 is constant or fixed and that the interface potential of the gate insulating layer 2 relative to the electrolyte solution 6 is expressed by Eg, the potential of the source 3 relative to the gate is expressed by Es, and the potential of the reference electrode 7 relative to the electrolyte solution 6 is expressed by Er. Since the characteristic of the gate insulating layer 2 varies depending on change in temperature T, the potential Eg changes in an amount expressed by .differential.Eg/.differential.T. In addition, since the electroconductivity of the electroconductive channel 5 in the field-effect transistor varies according to the operating current Id flowing from the drain to the source, the source potential Es is also affected by the temperature T, the amount of variation of the source potential Es as a result of change in temperature T being expressed by .differential.Es/.differential.T. Moreover, a similar description applies to the interface potential Er of the reference electrode 7 and the interface potential Er varies according to change in temperature T in an amount expressed by .differential.Er/.differential.T.
Accordingly, when variation in characteristic of the ion-sensitive FET transducer as a result of change in temperature is taken into consideration in the equation (1), the following equation can be obtained. EQU .differential.V/.differential.T=.differential.Eg/.differential.T+.different ial.Es/.differential.T-.differential.Er/.differential.T (2)
The amount of variation of the electroconductivity of the channel as a result of change in temperature, i.e., .differential.Es/.differential.T, varies also depending on the type of transducer and/or the method for the manufacture of the transducer. However, the amount of variation of the interface potential of the gate insulating layer 2 as a result of change in temperature i.e., .differential.Eg/.differential.T, corresponds to the term for temperature in the Nernst's equation. On the other hand, the temperature dependency of the interface potential of the reference electrode is discussed in details in, for example, Kagaku Binran, Kiso-hen II (Manual of Chemistry, Fundamental II), #9.9 Cell, 1966, edited by the Chemical Society of Japan, and is described as dependent on the material for the electrode and the concentration of ions in the solution contained therein. However, even if the material for the electrode and the concentration of ions in the solution are both fixed, the electroconductivity still varies depending on the method of the manufacture of the transducer to some extent.
In view of the fact that the transducer composed of the various elements each having its own characteristic variable depending on change in temperature has heretofore been used in the measurement of the ion activity, not only an accurate and precise measurement can hardly be achieved, but also compensation for variation in characteristic of the ion-sensitive FET transducer as a whole resulting from change in temperature cannot be achieved easily.
In order to obviate the above described disadvantage and inconvenience, the present inventors have tried the development of the ion-sensitive FET transducer wherein a diode is incorporated therein, taking advantage of the diode which is known as having a substantially fixed temperature dependency. During the measurement carried out by the use of the transducer or FET sensor wherein the diode is incorporated therein, the temperature measurement of the measurement system by the use of the diode as a temperature sensor is carried out at the same time so that, by using a value representative of the temperature dependency of the system including the FET sensor and the reference electrode which has previously been determined, an electric circuitry for the system is devised so as to achieve the compensation for variation in characteristic of the system. However, it has been found that various disadvantages described below are involved.
(a) Since the pattern for the FET manufacturing is complicated, the element tends to become bulky. PA1 (b) The number of lead wirings is increased. PA1 (c) Since the electric circuitry for the system must have a temperature detecting circuit, the electric circuitry for the sensor system tends to become complicated.
Moreover, although the temperature dependency of the diode can be controlled more easily than that of the field-effect transistor, errors tend to occur in the measurement because of different performance characteristics. In order for the error in the measurement to be negligible, not only must the diode be of high quality as compared with that generally used in electronic circuitries, but also the temperature dependency of the measurement system at the operating current for the FET sensor must be controlled to a very small value. By way of example, in the case where the temperature dependency of the measurement system at a certain operating current is 1 mV/.degree.C., while the temperature dependency of the diode is generally permitted within 0.15 mV/.degree.C., the pH measurement at ambient temperature and that at the body temperature will give a maximum difference of about 0.03.
In view of the above, in order to give a precise and accurate measurement according to the method described above, the measurement method requires a strict quality control, which is generally considered hard to achieve, and the employment of the unnecessarily complicated electric measuring circuitry.